1. Technical Field
The present invention relates to an electric power steering apparatus, a control method thereof and a computer readable medium storing a program.
2. Related Art
Recently, an electric power steering apparatus has been proposed, which includes an electric motor in a vehicle steering system and assists a driver with his or her steering force by use of power of the electric motor.
The electric power steering apparatus is controlled by a controller. To control the electric motor drive, the controller first sets a target current to be supplied to the electric motor in accordance with steering torque, vehicle speed, and the like. Then, the controller performs feedback control so that a deviation between the target current and an actual current would become zero, in order that the actual current actually passing through the electric motor may coincide with the set target current.
For example, an electric power steering apparatus disclosed in Patent Literature 1 performs a proportional action of multiplying, by proportional gain Kp, the current deviation between the target current and the actual current, and an integral action of multiplying, by integral gain Ki, the integral value obtained by integrating the current deviation.
Under the feedback control alone, however, response to a change in the current passing through the electric motor may not be necessarily said to be sufficient. Thus, there has been proposed an approach of feedforward control execution in which a motor drive signal increases in magnitude in accordance with the target current, in addition to the feedback control, to thereby heighten steering response. (See Patent Literature 2, for example.)
To control the drive of the electric motor, the controller, which controls the electric power steering apparatus, determines the current to be supplied to the electric motor on the basis of detected steering torque.
There has been proposed a technique in which, at the time of determination of the current to be supplied to the electric motor, which is based on the detected steering torque, a phase compensator is used to provide a phase compensation of the detected steering torque in order to enhance the stability of the steering system, and the current is supplied to the electric motor in accordance with the phase-compensated steering torque. (See Patent Literature 3, for example.)
Patent Literature 4 also proposes the following technique. Specifically, with the fact taken into account that oscillation occurs at a resonance frequency in a resonance system including a spring element that forms the torque sensor and the inertia of a steering wheel, this technique provides a band-elimination filter for eliminating resonance frequency components of a signal from a motor drive control system on the output side of a torque sensor.    Patent Literature 1: Japanese Patent Application Laid Open Publication No. 2001-315657    Patent Literature 2: Japanese Patent Application Laid Open Publication No. 10-100914    Patent Literature 3: Japanese Patent Application Laid Open Publication No. 10-167086    Patent Literature 4: Japanese Patent Application Laid Open Publication No. 2-164665
Here, characteristics of a system that performs the feedback control on the basis of the current deviation between the target current and the actual current are determined by characteristics of a transfer function of the actual current relative to the target current. For example, the stability of the system is determined by a denominator of the transfer function, and response to the actual current is determined by a numerator of the transfer function. Then, a system that performs the feedback control using the proportional gain Kp and the integral gain Ki is incapable of effecting a change only in any one of the denominator and numerator of the transfer function, regardless of how the proportional gain Kp and the integral gain Ki are changed. In other words, the system is incapable of setting the stability of the system and the responsiveness to the actual current independently of each other.
Therefore, an object of the present invention is to achieve an improvement in the responsiveness to the actual current without affecting the stability of the system.
Also, the improvement in the responsiveness is effected by performing the feedforward control in addition to the feedback control. However, even in a steady state, an adder that adds together a feedback control output and a feedforward control output has addition of the feedback control output and the feedforward control output, and thus, even in the steady state, a difference arises between the target current and the actual current. Hence, the mere addition of the feedforward control to the system that performs the existing feedback control might greatly affect stationary characteristics of the electric motor. Therefore, a novel system including the existing feedback control is required for the addition of the feedforward control without affecting the stationary characteristics of the electric motor.
Also, the electric power steering apparatus drives the electric motor in accordance with steering torque applied to the steering wheel, and transfers drive power of the electric motor to a pinion shaft that forms a pinion-and-rack mechanism. Further, the drive power of the electric motor is transferred to a rack shaft through the pinion shaft, and effects a rectilinear motion of the rack shaft thereby to change the direction of a wheel to be turned.
For suppression of vibration in the control system of the electric power steering apparatus, it is therefore important that the electric motor, the pinion shaft and the rack shaft also be considered inertial elements.